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CHAPTER XVI

THE FAMILY NUMMULINIDA

In this family the foraminiferal shell exhibits its highest structural development. One of the chief characters of three of its sub-families is the presence of a supplemental skeleton with a more or less complex interseptal canal system. The shell-wall is always finely perforated. The general plan of the shell is spiral and equilateral; some exceptions to this are Amphistegina, Archædiscus, and Faujasina: or cyclical, as in Cycloclypeus and Orbitoides. The tests in the NUMMULINIDE are either discoidal, lenticular, ovate, or fusiform in contour.

The isomorphous forms in this group which agree with those of other divisions according to some general plan of growth are quite numerous, and we may quote for examples Fusulina (=Alveolina) and Cycloclypeus (=certain species of Orbitolites).

From a geological point of view this family is perhaps the most important of the entire order, since nearly all its members are active agents in the formation of limestone rocks of considerable extent. Among the more noteworthy of these the genera Fusulina, Amphistegina, Operculina, Heterostegina, Nummulites, and Orbitoides may be mentioned.

To turn to the separate divisions of this family, the sub-family FUSULININE has the important genus Fusulina for its type. This, with the various allied genera and sub-genera, are fossil forms, and restricted to palæozoic strata. Their shell-structure is not typically nummuline, for the canal system and supplementary skeleton appear to be absent.

The next group of the POLYSTOMELLINE is a farther step towards the higher shell-structure seen in the Nummulites. In Nonionina the shell-wall is still simple in character, but the suture lines are sometimes thickened by an extra shell-layer. Polystomella comprises an instructive series of shells in which the graduations may be traced from those with a simple series of interseptal canals opening out at the suture lines by a row of pores, to those with a more complicated system, and with a varying amount of exogenous shell-structure forming a true supplemental skeleton, as well as secondary thickening of the test in the form of septal bridges laid at intervals across the sutural furrows, and also sutural costæ.

The genus Archædiscus represents the simplest form of the typical nummulite group, in being little more than a simple non-septate or partially septate tube wound round an axis which at intervals changes its direction, as in many of the milioline genera. This genus, which is restricted to Carboniferous rocks, shows an affinity with the nummuline forms in having a thick laminated shell. It is finely porous, but also exhibits a secondary kind of tubulation in the form of numerous coarser pores or canals. Am

phistegina has a more advanced type of shell nearer the true nummulites; in fact, it is difficult at times to be certain whether one is dealing with these forms or Nummulites when its chief character, the asymmetry of the test, is more or less obscure. Until lately it was held by Rhizopodists that Amphistegina had no interseptal canal system, but its presence has now been proved both in the fossil and recent examples.

Operculina is typically evolute and outspread, but the earlier chambers are embracing. Heterostegina is comparable with the last-named type, but the chambers are partitioned off into chamberlets.

The discoidal test of Nummulites consists of numerous chambers wound in a flat spiral, with the walls of the chambers double. The intermediate space is traversed by interseptal canals, which open out on the surface of the test. The walls of the chambers of each coil are so extended by alar prolongations or flaps that they become equitant and completely cover up the preceding whorl of the shell, and the test shows no signs of septation externally. The sinuous lines seen on the surface of Nummulites are the junctions of the alar extensions of the outer septal walls of the chambers.

The genera Cycloclypeus and Orbitoides, which constitute the sub-family CYCLOCLYPEINE, are examples of a cyclical method of growth in these shells. In the former genus there is a single layer of annular chambers divided into chamberlets, and with superimposed supplementary skeleton and numerous exo

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